Opponents accused Renzi of seeking to consolidate his grip on power via a system that will give the largest party to emerge from elections a winner's premium in terms of seats, guaranteeing a working majority in parliament.

Renzi has rebuffed charges of a power grab, saying Italy has to move towards something similar to the two-party systems in place in many other democracies and wave goodbye to decades of chronic political instability.

Two more confidence votes on separate articles of the law are expected on Thursday, before a final vote on the entire package next week.

If adopted, the new law is expected to come into force in 2016. It was approved by the upper house Senate in January.

The back-me-or-sack-me approach from Renzi is becoming a familiar one: it was also used to pilot labour market reforms through parliament last year.

Under his vision of Italy's constitutional future, governments will also not be encumbered by a Senate with extensive powers to block and delay legislation, as is currently the case.

Under constitutional reform also currently going through parliament, the upper chamber will become a much weaker body charged with representing the interests of Italy's regional bodies.